1. Field of the Invention
The present invention relates to an electromagnetic antilock brake system ("ABS") which comprises an electromagnetically actuated release arm capable of adjusting the position of a cam or other motion linkage to result in a controlled application of brake shoes to a brake drum. The invention further comprises a control system for controlling the actuation and release of the ABS.
2. Description of the Prior Art
Prior art drum brake systems comprise hydraulically actuated brake shoes which can be radially extended in order to exert a friction force against a rotating drum thereby decelerating and/or stopping a motor vehicle. Prior art ABS's comprise sensors, wheel brake cylinders, pressure modulator assemblies, a master cylinder, and an electronic control unit. Prior art wheel sensors comprise sensor units which are capable of sensing the wheel speed. Prior art pressure modulator assemblies may comprise solenoid valves, accumulators, damper chambers, orifices, and return pumps. Pressure modulators are expensive due to the number of expensive components which they comprise.
One or more prior art ABS's comprise one accumulator, one damper chamber, one orifice, and one return pump for both front brakes of a motor vehicle and one accumulator, one damper chamber, one orifice, and one return pump for both rear brakes of a motor vehicle.
In a prior art ABS, the wheel sensor detects the speed of wheel rotation. This speed is monitored by the electronic control unit. When the wheel speed decreases at a rate that would cause wheel lock up due to pressure being applied from the master cylinder, the electronic control unit uses the pressure modulator to release and reapply the pressure to the wheel cylinder to prevent complete wheel lockup.
In one or more prior art ABS's, the pressure modulator operates at a rate of 4 to 10 cycles per second (cps). In such systems, the accumulator absorbs the surge in hydraulic fluid; the damping chamber suppresses pressure oscillations; and the orifice acts as a flow restrictor. When the rate of deceleration decreases to an acceptable level, one of the solenoid valves opens such that pressure is reapplied from the master cylinder to the wheel cylinder.
In practice, current ABS's may not completely prevent wheel lockup. The wheels may alternate between lockup and rolling as the wheel cylinder pressure is modulated using prior art antilock braking systems. This wheel lock up decreases braking performance by providing a discontinuous braking torque. The frequency of modulation is limited by the response time of the mechanical parts used to control the wheel cylinder. The pulsation of fluid being pumped back to the master cylinder poses problems for some drivers who decrease pressure on the brake pedal when prior art antilock braking systems are activated. Such drivers often decrease this pressure in response to the pulsation of hydraulic fluid. The resulting decrease in pressure may turn off the antilock braking system and may reduce braking performance.
The present invention overcomes the problems of prior art antilock braking systems by employing a system that does not rely upon a pressure modulator and does not therefore result in the pulsation of hydraulic fluid present in prior art antilock braking systems.